The present invention relates to a reference potential generator for generating a predetermined reference potential, and more particularly, to a reference potential generator for generating a stable reference potential.
A reference potential generator is configured by a plurality of resistors or transistors that are connected in series between a power supply potential and a ground potential. The resistors or transistors divide the power supply potential and generate the divided potential as a reference potential. Such a reference potential generator is connected to a stage that precedes a control circuit of a voltage controlled oscillator (VCO), and supplies transistors in the control circuit with a constant reference potential. This keeps the operating speed of the control circuit constant.
FIG. 1 is a schematic circuit diagram of a prior art reference potential generator 100. The output terminal of the reference potential generator 100 is connected to the gate of a constant current source n-channel transistor 122 in a control circuit 120. The reference potential generator 100 includes a resistor 1 and a transistor 2, which are connected in series between a power supply potential VDD and a ground potential. The synthetic resistance of the resistance of the resistor 1 and the contact resistance of the transistor 2 divides the power supply potential VDD and generates a reference potential VR. The gate of the transistor 2 is connected to a node A between the resistor 1 and the drain of the transistor 2. The potential VA at node A is output as the reference potential VR.
FIG. 2 is a graph illustrating the relationship between the reference potential VR and the power supply potential VDD. When the power supply potential VDD is applied to the reference potential generator 100, the transistor 2 is activated. This causes current to flow from the power supply potential VDD to the ground potential VGND. As a result, the power supply potential VDD is divided by the contact resistance of the transistor 2 and the resistance of the resistor 1. This generates the reference potential VR, which has a constant potential difference VQ relative to the ground potential VGND. The reference potential VR may be adjusted by the resistance of the resistor 1 and the threshold value of the transistor 2. The reference potential VR controls the activation and deactivation of the n-channel transistor 122. Further, the reference potential VR controls the current flowing between the drain and the source of the activated n-channel transistor 122 at a constant value. That is, the reference potential VR is used as a regulated potential VW of the control circuit 120.
In the above reference potential generator 100, when the power supply potential VDD suddenly decreases due to a battery drain or the influence of noise, the reference potential VR, which is affected by the fluctuation of the power supply potential VDD, decreases. Referring to FIG. 2, for example, when the power supply potential VDD becomes lower than a predetermined potential vm, the reference potential generator 100 cannot maintain the regulated potential VW, which is required by the control circuit 120. This may cause the control circuit 120 to function erroneously.
It is an object of the present invention to provide a reference potential generator that maintains the regulated potential even when the power supply potential decreases.
To achieve the above object, the present invention provides a reference potential generator for generating a predetermined reference potential. The reference potential generator includes a first circuit including a first resistor and a first transistor connected in series between a first potential and a second potential. The first transistor has a first type of conductivity. The first circuit generates a first reference potential at a first node between the first resistor and the first transistor. An inverter is connected to the first node of the first circuit to generate an output potential that is substantially the same as either the first potential or the second potential in accordance with a potential difference between the first reference potential and the first potential. A second transistor is connected to an output of the inverter and has a second type of conductivity, which is opposite to the first type of conductivity. The second transistor includes a gate electrode, which is connected to the output of the inverter, a source, which is connected to the first potential, and a drain. A second resistor is connected to the drain of the second transistor. A second circuit includes a third resistor and a third transistor connected in series between the first potential and the second potential. The third transistor has the first type of conductivity. The second resistor is connected to a second node between the third resistor and the third transistor. The second circuit generates a second reference potential as the predetermined reference potential at the second node.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.